This invention relates to optical isolators for suppressing the light propagating backward along the optical path of an optical system.
When light generated, for example, by a semiconductor laser as a light source is transmitted through an optical fiber, the reflection of light takes place due to the dispersion of the light inside the optical fiber, local nonuniformity of the refractive index of the optical fiber, the connection of the optical fibers and the like, and the reflected light may propagate backward along the optical path and finally recombine with the semiconductor laser. Under such conditions, the oscillation of the semiconductor laser becomes unstable, which results in an undesirable increase in noise in the generated light. Optical isolators can be effectively used to suppress such reflected light.
FIG. 1 illustrates the structure of a conventional optical isolator. The light beam coming from a point "a" in FIG. 1 is converted into a parallel light beam by means of lens 1 and transmitted into a first beam splitter P.sub.1 serving as a polarizer. The polarizer P.sub.1 selectively permits the light polarized in a certain direction, e.g. the vertically polarized light to pass therethrough. The light passed through the polarizer P.sub.1 is then transmitted into a Faraday rotator FR which is constituted by a single crystal of a material such as YIG (an oxide containing Y.sub.2 O.sub.3, Fe.sub.2 O.sub.3 as major components), and passes through the Faraday rotator FR with its direction of polarization rotated by 45.degree.. The Faraday rotator FR is usually placed in a center space of a cylindrical magnet 3 as shown in FIG. 1, so that it is magnetized in a direction substantially parallel with the optical path. The light passed through the Faraday rotator FR goes into a second beam splitter P.sub.2 serving as an analyzer, whose polarization angle is deviated by 45.degree. from a vertical direction. Consequently, the light exiting from the Faraday rotator FR can pass through the analyzer P.sub.2 without further changing its plane of polarization, and is focused at point "b" after it has passed through lens 2. Therefore, if an end of an optical fiber is placed at point "b", the light coming from point "a" can be introduced into the optical fiber.
On the other hand, the light reflected in the optical fiber, etc. is transmitted from point "b" into the second beam splitter P.sub.2 which serves as a polarizer this time, via lens 2. The component of the reflected light polarized in the same direction as the polarization direction of polarizer P.sub.2 passes through the polarizer p.sub.2 and goes into the Faraday rotator FR. As is well known, the Faraday rotator FR changes the plane of polarization of light passing through it, depending upon the angle of the incident light relative to the direction of magnetization of the Faraday rotator. In the arrangement shown in FIG. 1 since the light is rotated by 45.degree. in the same direction as the incident light, the resultant direction of polarization of the light exiting leftward from the Faraday rotator FR is perpendicular to the direction in which the first beam splitter P.sub.1, which functions as an analyzer this time, permits a light to pass. Hence, the reflected light propagating backward from the Faraday rotator FR toward the semiconductor laser is blocked by the analyzer P.sub.1, thereby preventing the decrease of S/N ratio of the semiconductor laser, which would otherwise be caused by the recombination of the reflected light with the semiconductor laser.
In a conventional optical isolator as shown in FIG. 1, Rochon prisms or Glan-Thompson prisms are used as beam splitters serving as polarizers and analyzers P.sub.1, P.sub.2, respectively. The extinction ratios of these prisms are reportedly at most approximately 50 dB. The extinction ratios of the aforementioned single crystal materials such as YIG and GBIG (oxide consisting essentially of Gd.sub.2 O.sub.3, Bi.sub.2 O.sub.3, Y.sub.2 O.sub.3 and Fe.sub.2 O.sub.3) used for Faraday rotators are also reportedly about 40 dB. The loss in a reverse direction, or simply reverse loss of an optical isolator which comprises some of these components in combination is limited due to negative interference of the components. Thus, the reverse loss of such an optical isolator for single wavelength light is generally about 30 dB.
The study of coherent optical communications has been actively conducted recently, as optical communications are getting higher in density and speed. It has been pointed out that in such coherent optical communications the loss of an optical isolator in a reverse or backward direction would be insufficient if it is 30 dB or so, and that it must be at least double, namely, 60 dB or so. Therefore, a single-stage optical isolator as shown in FIG. 1 is insufficient, and so the use of a double-stage isolator is expected. In principle this makes it possible to double the reverse loss to 60 dB. However, when two such isolators are simply combined as in FIG. 2, the distance between the laser diode side "a" and the optical fiber side "b" becomes so great that the resultant coupling efficiency of the two isolators becomes extremely low. If the two isolators, i.e. two permanent magnets 3a and 3b, are approached each other in an effort to avoid this problem, the opposing faces of the magnets 3a, 3b attract each other and weaken the magnetic field to be applied to the Faraday rotators, though the overall size of the optical isolator is reduced. In some cases the rotators FR-1, FR-2 become unsaturated, so that they can no longer maintain their own isolating power.
In addition, the plane of polarization is rotated by 90.degree. from the incident light beam to the exiting light beam, so that the polarization angle of an optical system on the exit side "b" must be set perpendicular to that on the laser diode side "a".
Therefore, one way to increase the reverse loss of an optical isolator without suffering from the above problems is to improve the performance of each component in an optical isolator. Among others, the optimum size of the cylindrical serious attention to the fact that if the magnetic field applied to the Faraday rotator FR is nonuniform, the rotating power of the Faraday rotator differs from place to place in the above element, causing decrease of the extinction ratio. The extinction ratio also decreases if the magnetic field does not have sufficient intensity, since Faraday rotators become unsaturated.
To eliminate these problems, a large-scale Faraday rotator may be used, but it does not meet a requirement for the compact optical isolator. In order to obtain the best optical isolator, it is necessary to use as small a magnet as possible within a permissible range of extinction ratios.
In the conventional optical isolators, almost no attention has been paid to the optimum size of a permanent magnet for gaining a high reverse loss. It is, therefore, an object of this invention to determine the optimum size thereby providing a high-performance optical isolator comprising a cylindrical magnet having an optimum size.
Another way to increase the reverse loss of an optical isolator without suffering from the above problems is to provide a special construction of a multi-stage optical isolator.
Conventional multi-stage optical isolators are not only large in size but also disadvantageous in that the plane of polarization of light is rotated by 90.degree..
Thus, it is another object of this invention to solve these problems, thereby providing a compact, yet high-performance multi-stage optical isolator.